1. Field of the Invention
The present invention relates to a method of fabricating an integrated pressure sensor. More specifically, the invention relates to a method of fabricating an integrated pressure sensor by effecting trimming in a stage of the wafer.
2. Description of the Related Art
In conventional semiconductor pressure sensors, so far, the pressure sensitivity characteristics of the chips formed on a silicon wafer have been measured after the chips are separated into individual ones from the silicon wafer and are assembled on stems. FIG. 1 is a sectional view illustrating the structure when the pressure sensitivity characteristics of individual chips are to be measured relying upon a conventional method in a manner as described below. That is, a chip 121 separated from a silicon wafer is air-tightly joined to a glass 122 which is bored for introducing pressure and which is used for reducing thermal stress, and the glass is then air-tightly adhered to a bored stem 123 by using an adhesive agent 124. Then, gold wires or aluminum wires 126 from the chip 121 are bonded to external terminals 125 of the bored stem 123. Next, the bored stem 123 is air-tightly sealed with a cap 127 having a port. Finally, a pressure is introduced through the port 128 to measure the pressure sensitivity characteristics.
According to the above-mentioned prior art, the pressure sensitivity characteristics are measured by separating the chip 121 from the silicon wafer, assembling the chip on the bored stem 123, and applying a pressure thereto. Therefore, cumbersome operations and extended periods of time are required before measuring the pressure sensitivity characteristics after the wafer process has been finished. Moreover, chips having poor characteristics are often assembled. That is, in the prior art, the pressure sensors are adjusted for their sensitivities in the stage of chip making. It is necessary to solve the aforementioned problems.
In recent years, on the other hand, there has been a tendency to mount on a single chip a diaphragm portion which constitutes a semiconductor pressure sensor and a variety of operation processing units accompanied, however, by the need of trimming a plurality of sensor-constituting portions on which are mounted the diaphragm portion that constitutes the sensor portion and the operation processing units at the wafer stage in order to adjust the sensitivities so that the individual sensors in the form of chips will exhibit normal output values. That is, the integrated pressure sensors have heretofore been produced by forming a diaphragm, a piezo-resistance layer and a signal processing circuit on a silicon wafer for each of the sensor chips, joining the silicon wafer onto a seat, trimming the adjusting resistor under this condition, and dicing the silicon wafer into individual chips.
With the trimming being effected as mentioned above, however, the sensor chips after dicing produce fluctuating outputs being affected by distortion in the junction between the silicon wafer and the seat. That is, the distortion built up by the junction of the silicon wafer and the seat is released by dicing, whereby the stress in the diaphragm undergoes a change resulting in a fluctuation in the output after the dicing. Or, more specifically, the distortion that is built up due to a difference in the coefficient of thermal expansion between the silicon wafer and the seat when they are joined together, is released in a step of dicing. Therefore, the output fluctuates even if the adjustment is accomplished in the wafer stage.